The sealing materials in a fuel cell assembly, such as a solid oxide fuel cell (SOFC) device, are intended to provide a robust, hermetic seal between the frame of the fuel cell and the electrolyte contained in the fuel cell. The sealing material must be able to survive extended service at elevated temperatures and repeated thermal cycles. For example, fuel cell devices undergo large thermal cycling and are subject to large thermal gradients which may induce thermal stresses in the fuel cell components. If the sealing material expands at a rate that is different than the thermal expansion rate of the frame or the electrolyte sheet, the sealing material may either crack or cause cracking of the electrolyte sheet. Accordingly, sealing materials used in fuel cell frit seals need to be able to withstand high temperature fluctuations and have coefficients of thermal expansion (CTE) compatible with the electrolyte sheets used in the fuel cell as well as the material comprising the frame of the fuel cell.
In addition to these physical requirements, the sealing materials must meet certain chemical requirements. For example, the sealing materials must not degrade the electrical operation of the device, either through contamination of the solid oxide fuel cell cathode or contamination of other components contained in the fuel cell. Further, the sealing materials must be electro-chemically stable and not exhibit ion mobility under an applied electric field as this may lead to failures through the generation of oxygen blisters. Also, the sealing materials should exhibit acceptable flow characteristics in the desired sealing temperature range. Frit seal materials used to form hard seals should exhibit crystallization in the desired sealing range in addition to acceptable flow characteristics and mechanical robustness.
Despite these requirements, many material compositions presently used for glass frit sealing materials contain Li2O, B2O3 and/or other alkali oxides. Sealing materials containing these oxides may be contaminating, electro-chemically unstable, and/or lack robustness. More specifically, frit seal materials containing Li2O may lead to electrochemical decay while frit seal materials containing B2O3 may lead to cathode contamination. In either case, the performance of the fuel cell may be degraded based on the presence of such materials.
Accordingly, a need exists for improved glass frit-based sealing materials for solid oxide fuel cells.